a(n) is the number of ballot sequences of length n tied or won by at most 2 votes.

Terms

a(0) =1a(1) =2a(2) =4a(3) =6a(4) =14a(5) =20a(6) =50a(7) =70a(8) =182a(9) =252a(10) =672a(11) =924a(12) =2508a(13) =3432a(14) =9438a(15) =12870a(16) =35750a(17) =48620a(18) =136136a(19) =184756a(20) =520676a(21) =705432a(22) =1998724a(23) =2704156a(24) =7696444a(25) =10400600a(26) =29716000a(27) =40116600a(28) =115000920a(29) =155117520